A. WHY METALLIC BONDING
Metallic Powder Coatings can be produced to give a range of metallic finishes from silver
to gold and with all ranges of gloss and profile. By adding Aluminium, Bronze, Zinc,
Magnesium and Stainless steel pigments to the thermosetting Powder Coatings.
The most used pigment for silver metallic effect is Aluminium 1-2% in the leafing and
3-5% in the non-leafing form. With Aluminium pigments the fine Leafing grades will
produce the brightest silver and glossy effects these are the lower average particle size.
(4-12 microns) The Non- Leafing are higher average particle size (10-20 microns) and give
a sparkle effect.
Smooth Metallic effect Powder Coatings cannot be produced using the normal powder
coating process because the shear forces that occur during extrusion distort the metallic
particles causing them to loose their lustre. The extrusion process can produce some
excellent bright hammer finish effects but on the whole they tend to be silver and dark grey
Dry blending where the metal pigments are mixed with the thermosetting powder coatings
is a simple production method but can be highly dangerous process prone to explosion
with out the correct inerting methods and procedures. The finished powder will give a
brighter and glossy effect but the metallic pigment will tend to separate at the point of
application this in turn will cause pigment agglomeration and in the reclaim system cause
colour changes. Blended products cause major application problems because of the free
metal particles that exist within the mix including the clogging of guns during corona
The production of Metallic Powder Coatings is best achieved via the Bonding process that
is relatively safe and does not leave free metal particles within the powder when correctly
bonded. The Bonding process can be described as the complete attachment of metal
pigments to thermosetting Powder Coatings. The problems associated with separation and
agglomeration will disappear with reclaim equal to normal non-metallic powder coatings.
The metallic effect will be constant even with large batches.
The risks of incorporating the pigments, especially Aluminium. Can be eliminated by
determining a basis of safe operation for the process and using the best equipment for
Due to the specialist nature of this process only few Powder Coatings manufacturers have
moved into this process in house, with most choosing to use a Bonding service company
for Bonding the Metallic pigments into the base powder supplied by the Powder Coatings
The problem with any Bonding service is the time taken from sending the sample and
detailing the specification, identifying the required base Powder Coating, manufacturing
the base powder, having it Bonded and returning to the Powder Coating manufacturer and
on to the end user. A typical service would be eight weeks from receipt of colour pattern to
Bonded Powder Coatings.
Larger Bonding service companies are also the manufacturers of the metallic Pigments
that are used in the Bonding process so they can supply the raw materials for the Bonding
process. Main metallic pigment suppliers in Europe are Eckart and Benda-Lutz.
B. PRINCIPLES AND THEORY
The Bonding Process can be divided into three distinct parts :
1. Attachment - Attachment of the metal to the powder coating is achieved simply by heat
softening the powder coating and then mixing in the metallic pigment until all of the
particles are literally 'stuck' to the powder surface. The method most commonly used to
achieve attachment is high speed dispersing, the powder coating and the metal pigment
are loaded into a high speed mixer (normally jacketed to allow some control of process
temperature) and then mixed for some minutes at high speed. The energy of mixing
provides the required temperature rise for the powder to reach it's softening point
(40-600C), softening allows the metal pigment to adhere to the powder surface.
Mixing continues until all of the metal particles have adhered to the softened powder. The
end point is critical and is controlled via temperature control and energy instrumentation.
Once the end point has been reached the mix must be quickly discharged into a cooler to
prevent premature curing / solidification.
2. Cooling / Batching - The warm discharged mix has to be cooled as quickly as possible
(150C) and this is achieved using a separate low intensity mixer that is jacket cooled or
have sufficient cooling to reduce this bulk temperature. In most cases the bonded quantity
is a small part of a larger batch so blending of a number of mixes is also achieved at this
stage. The size of the cooler mixer is governed by the batch size spread.
3. Sieving - Because the powder coating is subjected to heat during the bonding process,
and as this can be localised, it is necessary to refine the final product to remove any
agglomerates that may have been formed. Course sieving at 130 - 150 Microns is usually
The final requirements of bonded powder coating are optimum powder reclaim due to the
fine pigments bonding to the larger powder coating particles. No separation from the
powder coating during spray application, even application of effect pigment and the
powder coating to the work piece.
C. BACKGROUND TO DUST EXPLOSIONS IN POWDER COATINGS AND METALLIC
Manufacturers that are considering manufacturing metallic effect powder coatings will
need to consider the process risks. The process will involve ‘bonding’ of fine aluminium
flake pigment onto the surface of warm thermoset powder coating particles. Fine
aluminium powder under the right conditions, has the potential to give rise to severe dust
explosion hazards. Powder coating particles themselves are also potentially flammable as
dusts. Therefore the handling and processing of these materials needs to be performed in
a way which minimises the risk of a dust explosion.
In order to define a 'basis of safety' for operation of a process, it is necessary to consider
the level of risk reduction afforded by any prevention or protection methods proposed.
This enables a judgement to be made on the adequacy of any proposed system for
reducing the risk to tolerable levels. Key information required for assessing risk in this
situation is explosion data for the materials being handled.
Both powder coatings and metal powders are potentially explosive if dispersed in air within
a certain concentration range. Ignition could result if an ignition source of sufficient igniting
power was simultaneously present.
The amount of energy required to ignite a dispersed dust cloud of aluminium is dependent
primarily upon the particle size distribution of the powder and to a lesser extent upon the
type of surface treatment. Other factors such as age, moisture content and particle shape
(i.e. sphere or flake) also play a role. The ease of ignition is defined by a parameter called
Minimum Ignition Energy (MIE) and literature values range from less than 1 mJ up to 50
mJ. Therefore, the fine leafing grades would be expected to have the lowest ignition
To put this into perspective, below is a table of possible ignition energies of electrostatic
discharges that might occur in a bonding plant.
|Spark from flange||0.5|
|Spark from a scoop/shovel||2 |
|Spark from a 200 litre metal drum||40 |
|Spark from a person ||10-30|
|Spark from large metal items||50-100|
|Brush discharge from non-conductors (e.g. large plastic bags, powder surfaces and plastic ducting)||3-5 |
Therefore, although ignition of a dust cloud by brush discharge hasn’t been achieved in
practice, it cannot be ruled out. Ignition of aluminium by electrostatic discharge from
isolated conductors or charged human operators is clearly possible. Probability of ignition
of aluminium from mechanical sparks, friction sparks and hot surface is more difficult to
predict, and depends upon many factors. However, it is safe to assume that ignition could
occur due to sparks say from an overheated mixer bearing or an overheated motor.
If an explosion were to occur, the explosion violence of an aluminium explosion can be
very high. The explosion violence is defined by a parameter called the Kst value, which is
a function of the maximum rate of pressure rise in an unvented explosion. Reported values
for pure aluminium powder are in the range 300-1000 bar.m/s. Even at the lower end of
this range, the explosion severity will be very high and therefore significant explosion
strength can be expected, even from quantities as small as 1kg. Aluminium explosions are
quite difficult to protect against and therefore focus is generally on prevention.
Powder Coatings themselves are potentially explosive dusts. Minimum ignition energy is
dependent upon particle size and to a lesser extent, on formulation. Fine powder particles
(dv,50 3-4 microns) have been measured as 1-3 mJ (reference 2). However, as it is
unlikely that powder particles of this fineness will be used as the base for bonding, a
typical MIE range would be 10-30 mJ. Again, Powder Coating particles are susceptible
to certain types of electrostatic discharges, the exception being brush discharges that
would not be expected to ignite Powder Coating particles. In terms of explosion severity,
powder coatings typical have a range of Kst values from 100-200 bar.m/s, 200 being only
for very fine psd material. An explosion would typically be significantly less violent than an
aluminium explosion, but nevertheless, could still cause significant damage.
When aluminium powder is mixed with normal powder coating, literature data suggests
that the Kst value of the powder coating increased by about 10% when 5-6% by weight
aluminium is added. Only when about 25% by weight Aluminium is added, does the Kst
approach that of pure aluminium powder. The minimum ignition energy of the powder
coating is only really decreased when greater than 10% of the finest leafing grades are
used. Therefore in many respects, the most hazardous part of Bonding, is in the handling
of the pure Aluminium. However, it should be noted, that segregation of the aluminium
could alter the above figures in favour of higher Kst values and lower MIE values.
D. BONDING PROCESS ROUTE
D.1 Bonding and cooling
In order to produce bonded metallic powder coatings, a process is required that will heat
the powder to its softening point, usually 45-550C. The heat input will be achieved by a
combination of mechanical energy from high speed mixing tools, from a hot water jacketed
mixer, or a combination of the two. The mixing speed will be variable, as generally high
speed is required to heat up the powder within a reasonable time scale, but low speed is
preferable for mixing in the pigment and achieving the adhesion. Low speed minimises
damage to the pigment structure. When bonding is achieved, it is important to cool the mix
as quickly as possible to avoid excessive agglomeration and lump formation. In the
extreme, bonded batches can completely solidify in the bonding mixer if overheated or not
cooled fast enough. Cooling will be achieved either by introducing chilled water into the
bonding mixer jacket, or cooling in a separate cooler mixer.
The fastest cooling is likely to be achieved by using the separate mixer/cooler
combination, as the single mixer/cooler is attempting to cool an already hot mixer.
However, as the mixer batch size becomes larger, cooling using a jacket (even in a
separate cooler mixer) can be very time consuming as the cooling surface area to batch
size ratio reduces. However, the advantage of a separate mixer/ cooler is that the bonding
and cooling cycles can be de-coupled, allowing simultaneous bonding/cooling, thus
increasing capacity. A possible disadvantage is the additional cleaning time, with two
rather than one mixer to clean. However, the cooler mixer should be relatively easier to
clean, as fusion of powder to its surfaces is unlikely.
An equation for estimating cooling time is as follows :
T = Time (secs),
M = mass of powder (kg)
A = jacket heat transfer area (m2)
U = Overall heat transfer coefficient (W/m2C)
Tw = Chilled water temperature (C)
Ts = Starting temperature (C)
Tf = required final temperature (C)